Reduced strain refractory ceramic composite and method of making

ABSTRACT

A composition is disclosed comprising a fine zircon component having a median particle size of less than 5 μm, a medium zircon component having a median particle size of from 5 μm to 15 μm, and a sintering aid, wherein the composition, after firing, has a strain rate of less than about 1×10 −6 /hr. A method for making a green body comprising contacting a fine zircon component having a median particle size of less than 5 μm, a medium zircon component having a median particle size of from 5 μm to 15 μm, and a sintering aid, and then forming the mixture into a desired shape is disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 12/692,220, filed Jan. 22, 2010, titled REDUCED STRAIN REFRACTORYCERAMIC COMPOSITE AND METHOD OF MAKING; which is a divisionalapplication of U.S. application Ser. No. 11/800,584, filed May 7, 2007,now U.S. Pat. No. 7,704,905, titled REDUCED STRAIN REFRACTORY CERAMICCOMPOSITE AND METHOD OF MAKING. This Applicant claims the benefit ofpriority under 35 US.C. §120 & 121 of the above-identified applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to refractory ceramic materials andspecifically, to the use of such materials in the manufacture of sheetglass by the fusion process.

2. Technical Background

The fusion process is one of the basic techniques used to produce sheetglass and can produce sheet glass having surfaces with superior flatnessand smoothness relative to sheet glass produced by alternativeprocesses, such as for example, the float and slot drawn processes. As aresult, the fusion process has found advantageous use in the productionof the glass substrates used in the manufacture of light emittingdisplays, such as liquid crystal displays (LCDs).

The fusion process, specifically, the overflow downdraw fusion process,includes a supply pipe which provides molten glass to a collectiontrough formed in a refractory body known as an isopipe. During theoverflow downdraw fusion process, molten glass passes from the supplypipe to the trough and then overflows the top of the trough on bothsides, thus forming two sheets of glass that flow downward and theninward along the outer surfaces of the isopipe. The two sheets meet atthe bottom or root of the isopipe, where they fuse together into asingle sheet. The single sheet is then fed to drawing equipment thatcontrols the thickness of the sheet by the rate at which the sheet isdrawn away from the root. The drawing equipment is located welldownstream of the root so that the single sheet has cooled and becomerigid before coming into contact with the equipment.

The outer surfaces of the final glass sheet do not contact any part ofthe outside surface of the isopipe during any part of the process.Rather, these surfaces only see the ambient atmosphere. The innersurfaces of the two half sheets which form the final sheet do contactthe isopipe, but those inner surfaces fuse together at the root of theisopipe and are thus buried in the body of the final sheet. In this way,the superior properties of the outer surfaces of the final sheet areachieved.

The dimensional stability of an isopipe during the glass forming processcan affect the overall success of the manufacturing process, as well asthe properties of the manufactured glass sheet. In the overflow downdrawfusion process, an isopipe can be subjected to temperatures of about1,000° C. While exposed to these temperatures, an isopipe must supportits own weight, the weight of the molten glass contained within theisopipe and overflowing its sides, and at least some tensional forcethat is transferred back to the isopipe through the fused glass as it isbeing drawn.

Commercial and market factors require a continuous increase in the sizeof light emitting displays and thus, the size of sheet glass. Dependingon the width of the sheet glass to be produced, an isopipe can have anunsupported length of about 1.5 meters or more.

To withstand these demanding conditions, isopipes have conventionallybeen manufactured from isostatically pressed blocks of refractorymaterial (hence the name “iso-pipe”). In particular, isostaticallypressed zircon refractories have been used to form isopipes for thefusion process. Conventional zircon refractories are comprised of ZrO₂and SiO₂, or equivalently ZrSiO₄. Even with such high performancematerials, isopipe materials can creep, resulting in dimensional changeswhich limit their useful life. In particular, isopipes exhibit sag suchthat the middle of the unsupported length of the pipe drops below theheight of its outer supported ends.

Thus, there is a need to address dimensional stability and othershortcomings associated with conventional isopipes and methods formanufacturing sheet glass. These needs and other needs are satisfied bythe composition and methods of the present invention.

SUMMARY OF THE INVENTION

The present invention relates to a refractory ceramic material that can,in one aspect, be used in the manufacture of sheet glass by, forexample, the overflow downdraw fusion process, and specifically to anisopipe designed to control sag during use. The present inventionaddresses at least a portion of the problems described above through theuse of a novel refractory ceramic composition and method of making.

In a first aspect, the present invention provides a compositioncomprising a fine zircon component having a median particle size of lessthan 5 μm, a medium zircon component having a median particle size offrom 5 μm to 15 μm, and a sintering aid, wherein the composition, afterfiring, has a strain rate of less than about 1×10⁻⁶/hr.

In a second aspect, the present invention provides a method of making agreen body comprising contacting a fine zircon component having a medianparticle size of less than 5 μm, a medium zircon component having amedian particle size of from 5 μm to 15 μm, and a sintering aid to forma mixture; and then forming the mixture into a desired shape.

In a third aspect, the present invention provides an article made by themethod described above.

Additional aspects and advantages of the invention will be set forth, inpart, in the detailed description, FIGURE, and any claims which follow,and in part will be derived from the detailed description or can belearned by practice of the invention. The advantages described belowwill be realized and attained by means of the elements and combinationsparticularly pointed out in the appended claims. It is to be understoodthat both the foregoing general description and the following detaileddescription are exemplary and explanatory only and are not restrictiveof the invention as disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing, which is incorporated in and constitutes apart of this specification, illustrates certain aspects of the presentinvention and together with the description, serves to explain, withoutlimitation, the principles of the invention.

The FIGURE is a schematic diagram illustrating a representativeconstruction for an isopipe for use in an overflow downdraw fusionprocess for making sheet glass, in accordance with one aspect of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be understood more readily by reference to thefollowing detailed description, drawings, examples, and claims, andtheir previous and following description. However, before the presentcompositions, articles, devices, and methods are disclosed anddescribed, it is to be understood that this invention is not limited tothe specific compositions, articles, devices, and methods disclosedunless otherwise specified, as such can, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular aspects only and is not intended to be limiting.

The following description of the invention is provided as an enablingteaching of the invention in its currently known aspects. To this end,those skilled in the relevant art will recognize and appreciate thatmany changes can be made to the various aspects of the inventiondescribed herein, while still obtaining the beneficial results of thepresent invention. It will also be apparent that some of the desiredbenefits of the present invention can be obtained by selecting some ofthe features of the present invention without utilizing other features.Accordingly, those who work in the art will recognize that manymodifications and adaptations to the present invention are possible andcan even be desirable in certain circumstances and are a part of thepresent invention. Thus, the following description is provided asillustrative of the principles of the present invention and not inlimitation thereof.

Disclosed are materials, compounds, compositions, and components thatcan be used for, can be used in conjunction with, can be used inpreparation for, or are products of the disclosed method andcompositions. These and other materials are disclosed herein, and it isunderstood that when combinations, subsets, interactions, groups, etc.of these materials are disclosed that while specific reference of eachvarious individual and collective combinations and permutation of thesecompounds may not be explicitly disclosed, each is specificallycontemplated and described herein. Thus, if a class of substituents A,B, and C are disclosed as well as a class of substituents D, E, and Fand an example of a combination aspect, A-D is disclosed, then each isindividually and collectively contemplated. Thus, in this example, eachof the combinations A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F arespecifically contemplated and should be considered disclosed fromdisclosure of A, B, and C; D, E, and F; and the example combination A-D.Likewise, any subset or combination of these is also specificallycontemplated and disclosed. Thus, for example, the sub-group of A-E,B-F, and C-E are specifically contemplated and should be considereddisclosed from disclosure of A, B, and C; D, E, and F; and the examplecombination A-D. This concept applies to all aspects of this disclosureincluding, but not limited to any components of the compositions andsteps in methods of making and using the disclosed compositions. Thus,if there are a variety of additional steps that can be performed it isunderstood that each of these additional steps can be performed with anyspecific aspect or combination of aspects of the disclosed methods, andthat each such combination is specifically contemplated and should beconsidered disclosed.

In this specification and in the claims which follow, reference will bemade to a number of terms which shall be defined to have the followingmeanings:

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “component” includes aspects having two or moresuch components, unless the context clearly indicates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance can or cannot occur, and that the description includesinstances where the event or circumstance occurs and instances where itdoes not. For example, the phrase “optional component” means that thecomponent can or can not be present and that the description includesboth aspects of the invention including and excluding the component.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

As used herein, a “wt. %” or “weight percent” or “percent by weight” ofa component, unless specifically stated to the contrary, refers to theratio of the weight of the component to the total weight of thecomposition in which the component is included, expressed as apercentage.

As used herein, the term “isopipe” refers to any sheet forming deliverysystem used in a fusion process which produces flat glass wherein atleast a part of the delivery system comes into contact with the glassjust prior to fusion, irrespective of the configuration or the number ofcomponents making up the delivery system.

As used herein, the term “pore” or “pores” refers to a vacancy or voidwithin and/or between grains of a refractory material. The term “pore”is intended to describe vacancies and/or voids of varying sizes, but isnot intended to describe inter-atomic spaces within a material.

As used herein, the term “strain” refers to the deformation of amaterial caused by a stress.

The following US patents and published applications describe variouscompositions and methods for manufacturing sheet glass, and they arehereby incorporated by reference in their entirety and for the specificpurpose of disclosing materials and methods relating to the formation ofrefractory ceramics, isopipes, and the manufacture of sheet glass: U.S.Pat. No. 3,338,696; U.S. Pat. No. 3,682,609; U.S. Pat. No. 3,437,470;U.S. Pat. No. 6,794,786; and Japanese Patent Publication No. 11-246230.

As briefly introduced above, the present invention provides a refractoryceramic body and a method for manufacturing an improved refractoryceramic body that, for example, can be useful as an isopipe in themanufacture of sheet glass. Specifically, the present invention providesan improved zircon composition and an isopipe formed from the inventivezircon composition. The isopipe of the present invention can haveenhanced dimensional stability and longevity over conventional isopipesused in the manufacture of sheet glass.

Although the compositions, refractory bodies, and methods of the presentinvention are described below with respect to the manufacture ofisopipes and sheet glass, it should be understood that the same orsimilar compositions and methods can be used in other applications wheredimensionally stable refractory materials are required. Accordingly, thepresent invention should not be construed in a limited manner.

With reference to the drawings, FIG. 1 illustrates a schematic of anisopipe, typical of those used in the manufacture of sheet glass by, forexample, the overflow downdraw fusion process. A conventional isopipeand sheet glass manufacturing system comprises a supply pipe 9 thatprovides molten glass to a collection trough 11 formed in a refractorybody 13 known as an isopipe. During operation, molten glass can flowfrom the supply pipe to the trough where it can overflow the top of thetrough of both sides, forming two sheets of glass that flow downward andthen inward along the outer surfaces of the isopipe. The two sheets meetat the bottom or root 15 of the isopipe where they can fuse togetherinto a single sheet. The single sheet is then fed to drawing equipment(represented by arrows 17), which controls the rate at which the sheetis drawn away from the root, and thus, the thickness of the sheet. Thedrawing equipment is typically positioned downstream of the root suchthat the formed sheet glass has sufficiently cooled and become rigidbefore contacting the equipment.

Conventional isopipes can be comprised of preformed, commerciallyavailable zircon materials (Ferro Corporation, Penn Yan, N.Y., USA).Commercially available zircon materials can be classified by particlesize and utilized to form an isopipe. The conventional zircon materialcan be formed into a desired shape, such as an isopipe, and fired,producing a polycrystalline refractory ceramic body. A challenge in theformation of such a refractory ceramic body is achieving a densestructure that is resistant to creep. Creep, as used herein, refers tothe tendency of a material to move or to deform to relieve a stress.Such deformation can occur as a result of long-term exposure to levelsof stress that are below the yield or ultimate strength of the materialand can be greater in materials that are subjected to heat for longperiods of time. Lowering the creep rate of a refractory material suchas, for example, an isopipe, can result in less sag during use. Creeprate can accelerate in low density or high grain-boundary refractorymaterials, such as those having large amounts of pores located at grainboundaries and/or triple points.

Creep can occur in various forms, such as Nabarro-Herring creep (stressdriven bulk diffusion within grains) and/or Cobble creep (grain-boundarydiffusion). Not wishing to be bound by theory, Nabarro-Herring creep canbe related to the concentration and size of pores within a material,such as within and/or between grains of a ceramic, and can beproportional to grain size. A reduction in the concentration and/or sizeof pores between grains of a ceramic material can result in increasedbulk density and increased creep resistance. Similarly, Cobble creep canbe related to mass transport phenomena occurring along grain boundariesof a polycrystalline material, and can also be inversely related tograin size.

Conventional zircon refractory ceramics comprise zircon materials havinglarge grain sizes so as to minimize grain boundaries, and thus Cobblecreep. Use of zircon materials having a larger grain size can reduce theeffects of Cobble creep, but can simultaneously result in an increase inthe concentration and size of pores within the refractory body. Such anincrease in the concentration and size of pores can result in decreasedbulk density and decreased strength of an isopipe.

While increased density can improve the strength and performance of arefractory ceramic body, such as an isopipe, high density alone does notnecessarily ensure adequate resistance to creep. To withstand thestresses and high temperatures of, for example, the glass formingprocess, for prolonged periods of time, a refractory ceramic body shouldalso exhibit a low strain rate.

Conventional isopipes are typically prepared using zircon materials andcan include substantial pores within their structure.

The present invention provides a composition having a fine zirconcomponent, a medium zircon component, and a sintering aid, together witha method for manufacturing a refractory ceramic composite that exhibitsa low strain rate. Zircon compositions in accordance with the presentinvention can provide refractory ceramic materials exhibiting few pores,high bulk densities, high strength, and low strain rates.

Zircon Components

The composition of the present invention, in one aspect, comprises afine zircon component, a medium zircon component, and a sintering aid.The composition can optionally comprise a coarse zircon component. Eachzircon component can have a median particle size and the particle sizedistributions of each component can overlap with the distributions ofone or more other components. The distribution of zircon particle sizemodes in the composition can comprise discrete modes, such as, forexample, a discrete bimodal composition, or a continuous mode, such as,for example, a continuous bimodal distribution. In one aspect, thedistribution is a continuous bimodal distribution where the fine zirconcomponent contributes a larger total volume fraction to the compositionthan the coarse component. The distribution of particle sizes in acomposition can be measured and the particle size distribution of one ormore zircon components analyzed using conventional de-convolutionalgorithms.

In various aspects, the fine zircon component can have a median particlesize of less than 5 μm, the medium zircon component can have a medianparticle size of from 5 μm to 15 μm, and the optional coarse zirconcomponent, if present, can have a median particle size greater than 15μm. The median particle sizes and amounts of each zircon component canvary depending upon the desired porosity, bulk density, and strength ofa refractory ceramic article made from the composition.

The fine zircon component of the present invention comprises, in variousaspects, from greater than 0 to less than about 80 wt. %, for example,about 0.1, 0.5, 1, 2, 5, 10, 20, 30, 40, 44, 48, 50, 52, 55, 58, 60, 65,70, 75, or 80 wt. % of the composition; from about 30 to about 70 wt. %,for example, about 30, 40, 44, 48, 50, 52, 55, 58, 60, 65, or 70 wt. %of the composition; or from about 40 to about 60 wt. %, for example,about 40, 44, 48, 50, 52, 55, 58, or 60 wt. % of the composition.

The fine zircon component of the present invention, in various aspects,has a median particle size of less than 5 μm, for example, about 4.9,4.7, 4.3, 4.0, 3.7, 3.5, 3.1, 2.8, 2.8, 2.5, 2.0, 1.8, 1.5, 1.3, 1.0,0.9, 0.7, 0.4, 0.2, or 0.1 μm. In other aspects, the fine zirconcomponent has a median particle size of from less than 5 μm to about 0.1μm, for example, about 4.9, 4.7, 4.3, 4.0, 3.7, 3.5, 3.1, 2.8, 2.8, 2.5,2.0, 1.8, 1.5, 1.3, 1.0, 0.9, 0.7, 0.4, 0.2, or 0.1 μm; from about 3 μmto about 0.1 μm, for example, about 3.0, 2.8, 2.8, 2.5, 2.0, 1.8, 1.5,1.3, 1.0, 0.9, 0.7, 0.4, 0.2, or 0.1 μm; or from 2 μm to about 0.1 μm,for example, about 2.0, 1.8, 1.5, 1.3, 1.0, 0.9, 0.7, 0.4, 0.2, or 0.1μm. In a specific aspect, the fine zircon component has a medianparticle size of about 1 μm.

The medium zircon component of the present invention comprises, invarious aspects, from greater than 0 to less than about 80 wt. %, forexample, about 0.1, 0.5, 1, 2, 5, 10, 20, 30, 40, 44, 48, 50, 52, 55,58, 60, 65, 70, 75, or 80 wt. % of the composition; from about 10 toabout 70 wt. %, for example, about 10, 20, 30, 40, 44, 48, 50, 52, 55,58, 60, 65, or 70 wt. % of the composition; or from about 20 to about 60wt. %, for example, about 20, 30, 40, 44, 48, 50, 52, 55, 58, or 60 wt.% of the composition.

The medium zircon component of the present invention, in variousaspects, has a median particle size of from 5 μm to 15 μm, for example,5.0, 5.2, 5.5, 5.8, 6.0, 6.3, 6.7, 7.1, 7.5, 7.8, 8.0, 8.5, 9.0, 9.4,9.8, 10.0, 10.6, 11.1, 11.7, 12.2, 12.6, 13.0, 13.5, 14.0, 14.6, or 15.0μm. In other aspects, the medium zircon component has a median particlesize of from 5 μm to about 10 μm, for example, 5.0, 5.2, 5.5, 5.8, 6.0,6.3, 6.7, 7.1, 7.5, 7.8, 8.0, 8.5, 9.0, 9.4, 9.8, or 10.0 μm. In aspecific aspect, the fine zircon component has a median particle size ofabout 7 μm.

The optional coarse zircon component of the present invention comprises,in various aspects when present, from greater than 0 to about 50 wt. %,for example, about 0.1, 0.5, 1, 2, 5, 10, 20, 30, 40, 44, 48, or 50 wt.% of the composition; or from about 10 to about 30 wt. %, for example,about 10, 12, 14, 18, 20, 22, 25, 27, or 30 wt. % of the composition. Ina specific aspect, the coarse zircon component comprises about 20 wt. %of the composition.

The optional coarse zircon component of the present invention, invarious aspects, has a median particle size of greater than 15 μm, forexample, 15.1, 15.3, 15.7, 16.0, 16.5, 17.0, 17.7, 18.2, 19.3, 20.0,20.4, 21.0, 21.5, 22.0, 22.6, 23.0, 23.4, 24.0, 24.5, 25, 28, 30, or 40μm. In other aspects, the coarse zircon component has a median particlesize of from greater than 15 μm to about 25 μm, for example, 15.1, 15.3,15.7, 16.0, 16.5, 17.0, 17.7, 18.2, 19.3, 20.0, 20.4, 21.0, 21.5, 22.0,22.6, 23.0, 23.4, 24.0, 24.5, or 25 μm. In a specific aspect, theoptional coarse zircon component has a median particle size of about 20μm.

The selection of particular zircon components and the amount of eachcomponent used in a composition can vary, provided that, when combinedwith a sintering aid and fired, the resulting refractory ceramic bodyhas a strain rate of less than about 1×10⁻⁶/hr.

In one aspect, the composition of the present invention comprises about30 wt. % of a fine zircon component having a median particle size ofabout 1 μm, about 50 wt. % of a medium coarse component having a medianparticle size of about 7 μm, and about 20 wt. % of a coarse zirconcomponent having a median particle size of about 20 μm. In anotheraspect, the composition comprises about 40 wt. % of a fine zirconcomponent having a median particle size of about 1 μm, about 40 wt. % ofa medium coarse component having a median particle size of about 7 μm,and about 20 wt. % of a coarse zircon component having a median particlesize of about 20 μm. In another aspect, the composition comprises about50 wt. % of a fine zircon component having a median particle size ofabout 1 μm, about 30 wt. % of a medium coarse component having a medianparticle size of about 7 μm, and about 20 wt. % of a coarse zirconcomponent having a median particle size of about 20 μm. In yet anotheraspect, the composition comprises about 60 wt. % of a fine zirconcomponent having a median particle size of about 1 μm, about 20 wt. % ofa medium coarse component having a median particle size of about 7 μm,and about 20 wt. % of a coarse zircon component having a median particlesize of about 20 μm.

The distribution of particle sizes within each component is not requiredto be uniform. For example, a zircon composition can comprise a coarse,a medium, and a fine particle size zircon component. The coarse zirconcomponent can comprise a distribution wherein about 90 wt. % of thecoarse component has a particle size from greater than about 15 μm toabout 25 μm, and wherein about 10 wt. % of the coarse component has aparticle size greater than about 25 μm. The fine zircon component cancomprise a distribution wherein about 90 wt. % of the fine zirconcomponent has a particle size from greater than about 0.8 μm to about1.6 μm, and wherein about 10 wt. % of the fine zircon component has aparticle size greater than about 1.6 μm.

Individual zircon components can be purchased commercially (FerroCorporation, Penn Yan, N.Y., USA) or prepared from other zirconmaterials by, for example, grinding a commercially available zirconmaterial to a target median particle size. Such zircon components can beground by any method suitable for providing the desired median particlesize and distribution. In one aspect, a commercially available zirconmaterial is ball-milled with yttria-stabilized zirconia grinding mediato a desired median particle size. Components can be further ground, ifrequired, by wet-grinding in a solvent, such as, for example, methanol.

The particle size distribution of a zircon component can vary dependingupon the type and extent of grinding. For example, moderate grinding toa median particle size greater than about 2 μm can provide a broadparticle size distribution, whereas grinding to a median particle sizeof about 1 μm can provide a narrow particle size distribution.

A zircon material can also be classified and/or separated into one ormore particle size fractions by, for example, sieving a ground zirconcomponent. Grinding and particle sizing techniques are known and one ofskill in the art could readily select an appropriate zircon material andgrinding technique.

Sintering Aid

The composition of the present invention comprises at least onesintering aid. The sintering aid can be, in various aspects, from about0.05 to about 5 wt. % of the composition, or 0.1, 0.2, 0.5, 0.9, 1, 1.3,1.8, 2, 2.5, 3, 4, or 5 wt. % of the composition; or from about 0.1 toabout 0.6 wt. %, for example, about 0.1, 0.2, 0.3, 0.4, 0.5, or 0.6 wt.% of the composition. In various specific aspects, the compositioncomprises 0.2 wt. % or 0.4 wt. % of a sintering aid.

Sintering aids can comprise any material capable of mineralizing zircon,such as, for example, oxides and/or salts of titanium, iron, calcium,yttrium, niobium, neodymium, glass compounds, or a combination thereof,provided that, when contacted and/or mixed with the zircon componentsand fired, the resulting refractory ceramic body has a strain rate ofless than about 1×10⁻⁶/hr. In one aspect, the sintering aid is atitanium containing compound. In a specific aspect, the sintering aid isa titanium dioxide. The use of and amount of a particular sintering aidcan vary depending upon, for example, the size and mixture of zirconcomponents and the method utilized for forming the composition into adesired shape. If a sintering aid does not yield a fired article havinga strain rate of less than about 1×10⁻⁶/hr, the amount and/orcomposition of the zircon components and/or the sintering aid can bevaried, such as, for example, by using a titanium containing sinteringaid and/or increasing the concentration of the fine zircon component inthe composition.

One or more sintering aids of the same or different compositions can beused. A sintering aid can be contacted with and/or mixed with the zirconcomponents using any suitable method. In one aspect, the sintering aidis dry blended with the zircon components and isopressed. In anotheraspect, the sintering aid is mixed with the zircon components as aslurry, ball-milled to homogenize the resulting mixture, dried, andisopressed. The isopressed material can subsequently be fired to producea cured ceramic article. In one aspect, the sintering aid is added tothe zircon components in a highly dispersed form. In another aspect, themixture of sintering aid and zircon components is processed to achieve auniform distribution of the sintering aid and zircon components. Ahighly dispersed and/or uniformly mixed sintering aid can improve thedensity and/or strain rate of a subsequently fired ceramic article. Acomposition comprising a highly dispersed sintering aid can achieve atarget density and/or strain rate using significantly less sintering aidthan a similar composition comprising a poorly dispersed sintering aid.Sintering aids are commercially available (Sigma-Aldrich, St. Louis,Mo., USA) and one of skill in the art could readily select anappropriate sintering aid for a desired composition or ceramic article.

Zircon/Sintering Aid Composition

The components (i.e., zircon components and sintering aid) of thecomposition can be mixed by any suitable method, such as, for example,dry blending. It is preferable that the components of the composition beuniformly or substantially uniformly mixed. Uniform mixtures of multiplecomponents can provide ceramic articles having higher bulk densities,greater strength, and reduced strain. Such uniform mixtures can beachieved using conventional mixing and dispersion techniques. Mixingand/or dispersion of components can be performed by, for example, a highshear mixer such as a ball mill, an attrition mill, and/or a hammermill. An exemplary mixing process can be performed with a Processall®mixer, available from Processall Incorporated, Cincinnati, Ohio, USA. Ahigh shear mixer, such as a Processall mixer, is preferred in order toobtain a homogeneous blend of components. In one aspect, the multiplecomponents are mixed to provide a substantially homogeneous mixture.Such a homogeneous mixture can comprise a uniform or substantiallyuniform distribution of, for example, coarse, medium, and fine zirconcomponents and a sintering aid. Various mixing and dispersion techniquesare known in the ceramics and fine particle industry and one of skill inthe art could readily select an appropriate mixing and/or dispersiontechnique.

In one aspect, the composition of the present invention comprises about30 wt. % of a fine zircon component having a median particle size ofabout 1 μm, about 49.8 wt. % of a medium coarse component having amedian particle size of about 7 μm, about 19.8 wt. % of a coarse zirconcomponent having a median particle size of about 20 μm, and about 0.4wt. % of a TiO₂ sintering aid. In another aspect, the compositioncomprises about 40 wt. % of a fine zircon component having a medianparticle size of about 1 μm, about 39.9 wt. % of a medium coarsecomponent having a median particle size of about 7 μm, about 19.9 wt. %of a coarse zircon component having a median particle size of about 20μm, and about 0.2 wt. % of a TiO₂ sintering aid.

Forming and Firing a Ceramic Article

After mixing, a composition can be formed into a green body of anydesired shape, such as an isopipe, by a suitable technique, such as, forexample, slip casting, extrusion, isostatic pressing, and/or injectionmolding. A green body, as used herein, comprises a formed, but unfiredceramic material. Depending on the specific forming technique employed,liquids, solvents, and/or forming aids can optionally be mixed with thezircon composition to facilitate the forming process. Such liquids,solvents, and/or forming aids, if present, can comprise any materialsuitable for facilitating the forming process. In one aspect, theliquid, solvent, and/or forming aid, if present, comprises at least oneof methyl cellulose, water, glycerol, or a combination thereof. Theseliquids, solvents, and/or forming aids can be removed prior to or duringthe firing process or can remain in an article after firing. In oneaspect, a slip casting technique is utilized to form a high liquidcontent mixture comprising the composition into a desired shape. Inanother aspect, an extrusion technique is utilized to form thecomposition into a desired shape. In yet another aspect, an isostaticpressing technique is utilized to form a dry or substantially drycomposition into a desired shape. In an exemplary isostatic pressingtechnique, the pre-fired composition is optionally subjected to atapping and/or vacuum step to achieve a high degree of compaction atambient conditions, and then is isostatically pressed at about 18,000psi for a period of from about 5 to about 20 minutes. Forming techniquesare known in the ceramic industry and one of skill in the art couldreadily select an appropriate forming technique for a desired ceramicarticle.

Thereafter, the refractory can be prepared in accordance with techniquescurrently known in the art of with improved techniques which may bedeveloped in the future. The refractory can be fired to sinter at leasta portion of the zircon components of the composition. A firing step cancomprise heating the formed green body at a time and temperaturesuitable to form a stable refractory ceramic body. In one aspect, thefiring step can comprise heating a formed green body in an electricalfurnace at a temperature of from about 1,400° C. to about 1,650° C. fora period of from about 1 to about 48 hours. In another aspect, thefiring step can comprise heating a formed green body in an electricalfurnace at a temperature of from about 1,400° C. to about 1,600° C. fora period of from about 2 to about 24 hours. The firing step can beperformed in an air atmosphere, under an inert atmosphere, such ashelium, or under vacuum. Firing techniques for refractory ceramics areknown and one of skill in the art would readily be able to select andperform an appropriate firing step for a refractory ceramic compositionof the present invention.

Fired Refractory Ceramic Body

A fired refractory ceramic body prepared from the composition and methodof the present invention can exhibit low porosity, high bulk density,and low strain. Depending on the specific zircon composition, degree ofmixing, forming and firing technique, a refractory ceramic body preparedin accordance with the present invention can, in various aspects,comprise a bulk density greater than about 4.25 g/cc, 4.3 g/cc, 4.4g/cc, 4.5 g/cc, or more. The theoretical maximum bulk density for azircon article is about 4.63 g/cc. Thus, it is possible to achieve bulkdensity values of, for example, 90%, 92%, 94%, or 96% of the theoreticalmaximum.

The strength of resulting refractory ceramic body and its resistance tocreep and/or sag is dependent, in part, on the amount of pore spaceremaining in the refractory ceramic body. A refractory ceramic bodyhaving less pore space volume within its structure will generallyexhibit a greater resistance to creep than a body with greater porespace volume. Zircon refractory ceramic bodies prepared in accordancewith the present invention can have porosity values less than about 25%,less than about 12%, less than 10%, or less than about 3%.

The strength of a refractory ceramic body can be ascertained bydetermining the modulus of rupture (MOR) by, for example, ASTM C158. MORrefers to the amount of force needed to break a test sample and isusually expressed in pounds of force per square inch. The MOR of zirconrefractory ceramic articles prepared in accordance with the presentinvention can be greater than about 10×10³ psi, greater than about15×10³ psi, or greater than about 20×10³ psi. Such high strength (MOR)provides increased creep resistance to an article, such as an isopipe,during operation.

Increasing the density of a refractory ceramic body can improve sagand/or creep resistance, but further improvements can be made if therefractory ceramic body exhibits a low strain rate. A ceramic bodyhaving a reduced strain rate can be prepared with the zircon componentsand sintering aid of the present invention. The strain rate of arefractory ceramic body prepared in accordance with various aspects ofthe present invention can be less than about 1.0×10⁻⁶/hr, for example,about 8.5, 7.1, 6.4, 5.8, 5.5, 5.1, 4.8, 4.4, 4.3, or 3.8×10⁻⁷/hr. Inone aspect, the strain rate of a refractory ceramic body prepared inaccordance with the present invention can be less than about 50%, orless than about 25% of the strain rate of a conventional (isopressed)zircon isopipe.

Although several aspects of the present invention have been illustratedin the accompanying drawings and described in the detailed description,it should be understood that the invention is not limited to the aspectsdisclosed, but is capable of numerous rearrangements, modifications andsubstitutions without departing from the spirit of the invention as setforth and defined by the following claims.

EXAMPLES

To further illustrate the principles of the present invention, thefollowing examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how thearticles, devices, and methods claimed herein are made and evaluated.They are intended to be purely exemplary of the invention and are notintended to limit the scope of what the inventors regard as theirinvention. Efforts have been made to ensure accuracy with respect tonumbers (e.g., amounts, temperatures, etc.); however, some errors anddeviations should be accounted for. Unless indicated otherwise,temperature is ° C. or is at ambient temperature, and pressure is at ornear atmospheric. There are numerous variations and combinations ofprocess conditions that can be used to optimize product quality andperformance. Only reasonable and routine experimentation will berequired to optimize such process conditions.

Example 1 Preparation of Zircon Compositions

In a first example, a series of zircon compositions were prepared bymixing zircon materials of varying particle sizes, as detailed in Table1, below. Samples are identified by the weight percents of eachcomponent, followed by the median particle size of each component (e.g.,sample F comprises 10 wt. % of a coarse zircon component having a medianparticle size of 20 μm, 50 wt. % of a medium zircon component having amedian particle size of 7 μm, 40 wt. % of a fine zircon component havinga median particle size of 1 μm, and 0.20 wt. % of a TiO₂ sintering aid).Isopropyl alcohol was then added to each mixture to form slurries, whichwere then shear-mixed by ball milling for 2 hours. Sample B represents acomparative 7 μm zircon sample without a fine particle size zirconcomponent. Each composition was dried, isopressed and fired atapproximately 1,580° C. The density of each dried, isopressed, and firedsample are detailed in Table 1.

TABLE 1 Density of Zircon Compositions Sample Composition Sintering AidDensity, g/cc A Commercial Zircon 0.4 wt. % TiO₂ 4.1 B 7 μm none 3.84 C50/50 7:1 μm none 4.22 D 50/50 7:1 μm 0.4 wt. % TiO₂ 4.58 E 10/45/4520:7:1 μm none 4.29 F 10/50/40 20:7:1 μm 0.2 wt. % TiO₂ 4.44 G 20/60/2020:7:1 μm 0.4 wt. % TiO₂ 4.23 H 20/50/30 20:7:1 μm 0.2 wt. % TiO₂ 4.25 I20/40/40 20:7:1 μm 0.2 wt. % TiO₂ 4.24 J 20/40/40 20:7:1 μm 0.4 wt. %TiO₂ 4.41 K 20/30/50 20:7:1 μm 0.2 wt. % TiO₂ 4.36 L 20/30/50 20:7:1 μm0.2 wt. % TiO₂ 4.41 M 20/30/50 20:7:1 μm 0.4 wt. % TiO₂ 4.57 N 20/30/5020:7:1 μm 0.2 wt. % ZnO 4.29 O 20/20/60 20:7:1 μm 0.4 wt. % TiO₂ 4.62

The density values listed in Table 1 illustrate the increased densityobtainable with the use of fine, medium, and optional coarse zirconcomponents and/or sintering aids. The density of Samples D, M, and Oapproach the theoretical maximum density for zircon of 4.63 g/cc, whilethe commercial zircon and single particle size zircon of samples A andB, respectively, have significantly lower densities. The improveddensity achievable with addition of a sintering aid is illustrated inthe comparison of samples C and D.

Example 2 Improved Strain Rate of Zircon Articles

In a second example, the strain rate of samples prepared in Example 1were determined at 1,000 psi, 1,180° C., over 100 hours. The strain rateresults are detailed in Table 2, below.

TABLE 2 Strain Rate of Zircon Samples Sample Composition Sintering AidStrain Rate/hr A Commercial Zircon 0.4 wt. % TiO₂ 1.06 × 10⁻⁶ B 7 μmnone  5.9 × 10⁻⁶ C 50/50 7:1 μm none 1.30 × 10⁻⁶ D 50/50 7:1 μm 0.4 wt.% TiO₂ 4.21 × 10⁻⁷ E 10/45/45 20:7:1 μm none 1.60 × 10⁻⁶ F 10/50/4020:7:1 μm 0.2 wt. % TiO₂ 8.20 × 10⁻⁷ G 20/60/20 20:7:1 μm 0.4 wt. % TiO₂5.97 × 10⁻⁷ H 20/50/30 20:7:1 μm 0.2 wt. % TiO₂ 5.75 × 10⁻⁷ I 20/40/4020:7:1 μm 0.2 wt. % TiO₂ 8.43 × 10⁻⁷ J 20/40/40 20:7:1 μm 0.4 wt. % TiO₂4.34 × 10⁻⁷ K 20/30/50 20:7:1 μm 0.2 wt. % TiO₂ 4.32 × 10⁻⁷ L 20/30/5020:7:1 μm 0.2 wt. % TiO₂ 4.88 × 10⁻⁷ M 20/30/50 20:7:1 μm 0.4 wt. % TiO₂4.34 × 10⁻⁷ N 20/30/50 20:7:1 μm 0.2 wt. % ZnO 5.13 × 10⁻⁶ O 20/20/6020:7:1 μm 0.4 wt. % TiO₂ 4.84 × 10⁻⁷

As illustrated in Table 2, the strain rate of a zircon compositionprepared in accordance with the present invention, such as sample D, canhave a strain rate of about 36.8% less than that of a sample preparedwith a standard commercially available zircon alone. Sample E did notcontain a sintering aid and exhibited a strain rate equivalent to, orhigher than a commercial zircon material. Similarly, Sample N contained0.2 wt. % ZnO, but did not contain a sintering aid in accordance withthe present invention. Despite a high density (4.29 g/cc), Sample Nexhibited a strain rate over an order of magnitude greater than asimilar sample (Sample M) that contained 0.4 wt. % TiO₂, in accordancewith the present invention. The effect of varying amounts of sinteringaid is illustrated in Samples I and J, where the strain rate was reducedby approximately 50% with the use of a larger quantity of TiO₂ sinteringaid.

Various modifications and variations can be made to the compositions,articles, devices, and methods described herein. Other aspects of thecompositions, articles, devices, and methods described herein will beapparent from consideration of the specification and practice of thecompositions, articles, devices, and methods disclosed herein. It isintended that the specification and examples be considered as exemplary.

1. A method for making a green body comprising: a) forming a mixtureincluding: i) about 10 wt. % to 60 about wt. % of a fine zirconcomponent having a median particle size of less than 5 μm, ii) about 10wt. % to about 60 wt. % of a medium zircon component having a medianparticle size of from 5 μm to 15 μm, and iii) a sintering aid; and thenb) forming the mixture into a desired shape.
 2. The method of claim 1,wherein the mixture formed in step a) further comprises a coarse zirconcomponent having a median particle size of greater than 15 μm.
 3. Themethod of claim 1, wherein the sintering aid comprises at least oneoxide or salt of titanium, iron, calcium, yttrium, niobium, neodymium,or a combination thereof.
 4. The method of claim 1, wherein the step b)comprises an isostatic pressing process.
 5. The method of claim 1,further comprising a step of c) firing the desired shape at a time andtemperature sufficient to form an article having a strain rate of lessthan about 1×10⁻⁶/hr.
 6. The method of claim 1, wherein step a) furthercomprises, contacting at least one of the fine zircon component, themedium zircon component, or the sintering aid with at least one ofmethyl cellulose, water, glycerol, or a combination thereof.
 7. Aceramic green body made by the method of claim
 4. 8. The method of claim1, wherein step a) comprises forming a mixture including: i) about 10wt. % to 60 about wt. % of a fine zircon component having a medianparticle size of from about 0.1 μm to about 2 μm, ii) about 10 wt. % toabout 60 wt. % of a medium zircon component having a median particlesize of from 5 μm to 15 μm, and iii) a sintering aid.
 9. The method ofclaim 2, wherein the sintering aid comprises a titanium containingcompound.
 10. The method of claim 2, wherein the sintering aid is fromgreater than 0 wt. % to about 1 wt. % of the composition.
 11. The methodof claim 2, wherein the sintering aid is from greater than 0 wt. % toabout 0.5 wt. % of the composition.
 12. The method of claim 2, whereinthe fine zircon component comprises from about 40 wt. % to about 60 wt.% of the composition.
 13. The method of claim 2, wherein the fine zirconcomponent has a median particle size of from about 0.1 μm to about 2 μmand the coarse zircon component has a median particle size of from 15 μmto about 25 μm.
 14. The method claim 8, wherein the fine zirconcomponent comprises from about 40 wt. % to about 60 wt. % of thecomposition; and the mixture formed in step a) further includes a coarsezircon component having a median particle size of from 15 μm to about 25μm; and wherein the sintering aid comprises from greater than 0 wt. % toabout 0.5 wt. % of the composition and comprises a titanium containingcompound.
 15. The article method of claim 5, having a bulk density ofgreater than about 4.25 g/cc.
 16. The method of claim 5, wherein step b)comprises forming the mixture into the shape of an isopipe, and in stepc) the ceramic article is an isopipe.
 17. The method of claim 2, whereinin step a) at least the fine zircon component, the medium zirconcomponent, and the sintering aid are substantially uniformly mixed. 18.The method of claim 16, wherein the isopipe has a strain rate of lessthan about 1×10⁻⁶/hr.